Changing topics is a common behavior in human conversations [1]}. Recently, there is an increasing interest in building conversational systems that can proactively lead human-machine conversations via topic shifting [2]}, [3]}, [4]}, [5]}. Different from the methods of asking clarifying questions (Section REF ) and suggesting useful questions (Section REF ) which aim to follow, clarify, or further a user's original search topic to get more precious and fruitful results, a topic shifting module might suggest a new topic that changes the user's search intent.
